This is how I'm setting this up:
- there's an NSObject subclass (call it MyDialog) containing
IBOutlets for all the controls I care about.
- there's another NSObject subclass (call it SliderHelper) which is
intended to automate some behaviour common to a few controls in the
dialog (and other dialogs in future)
- MyDialog instantiates a SliderHelper and passes it an NSSlider*
outlet referring to a slider
- SliderHelper sets up the target and action for the slider
programmatically in an +initWithSlider: method..

> I'm trying to set the action for an NSSlider in a little helper class that
> is not the "file's owner" class associated with the window in the nib. At
> runtime, I get these console messages:
>
> *** +[SliderHelper myAction:]: unrecognized selector sent to class
> 0xeb74380
> HIToolbox: ignoring exception '*** +[SliderHelper myAction:]: unrecognized
> selector sent to class 0xeb74380' that raised inside Carbon event dispatch
>
> This is how I'm setting this up:
> - there's an NSObject subclass (call it MyDialog) containing IBOutlets for
> all the controls I care about.
> - there's another NSObject subclass (call it SliderHelper) which is
> intended to automate some behaviour common to a few controls in the dialog
> (and other dialogs in future)
> - MyDialog instantiates a SliderHelper and passes it an NSSlider* outlet
> referring to a slider
> - SliderHelper sets up the target and action for the slider
> programmatically in an +initWithSlider: method..
>
> +(id) initTestSlider:(NSSlider*)aSlider {
> SliderHelper* newOne = [[SliderHelper alloc] init];
> [newOne setSlider:aSlider];
> [[newOne slider] setTarget:self];

-init* is always an instance method, and always called as part of a [[Foo
alloc] initBlah] pair. Above, you're passing a Class object to setTarget,
but targets must be instances - hence the "unrecognized selector" error when
you try to send the action to the target.